Ceramic compositions which form dielectrics upon sintering are widely used for forming various electronic parts such as capacitors and piezoelectric parts. Among these electronic parts, electronic parts of ceramic-multilayer type such as a multilayer ceramic capacitor and a multilayer ceramic actuator are being developed at a high speed in recent years. For manufacturing an electronic part of this type, a ceramic composition in a powder form is formed into a sheet, followed by coating the sheet with an electrode paste acting as an inner electrode. Then, a plurality of such ceramic/paste laminates are superposed one upon the other, followed by compressing the superposed body and subsequently sintering the superposed body.
A ceramic material containing barium titanate as a main component is known in the art as a material used for manufacturing an electronic part of this type. However, such a high temperature as 1300.degree. to 1400.degree. C. is required for sintering barium titanate. Naturally, it is necessary for the metal used as the inner electrode, which is sintered together with the ceramic material, not to be oxidized and converted into an insulating material at the high sintering temperature, or not to be melted at the high sintering temperature. The inner electrode metal, which meets the requirement noted above, is restricted to noble metals such as gold, platinum and palladium, which are costly. It follows that the electronic part thus manufactured can be used in only the field in which the high manufacturing cost is not of importance.
If the sintering temperature of the ceramic composition can be lowered, it may be possible to widen the scope of selection of the metal which can be used as an inner electrode of a ceramic electronic part, leading to a low manufacturing cost of the electronic part. For example, if a ceramic composition which can be sufficiently sintered at 1050.degree. C. or less has been developed, it is possible to use a 80% silver/20% palladium alloy in place of a 100% palladium. This alloy is available at a cost less than 1/20 the cost of 100% palladium.
In recent years, vigorous studies are being made on ceramic compositions which can be sintered at a relatively low temperature, particularly, on compositions which provide dielectrics of a complex perovskite structure which contains lead in A-site upon sintering. Such compositions are also practically used. The ceramic compositions of this type include, for example, an oxide composition which can be converted upon sintering into a dielectric of the perovskite structure represented by the formula: EQU Pb(B.sub.1,B.sub.2)O.sub.3
where B.sub.1 is selected from Mg, Zn, Ni, Co, Fe, Mn and In, and B.sub.2 is selected from Nb, W, Ta and Sb. This ceramic composition can be sintered at 900.degree. to 1100.degree. C., thus making it possible to use a Ag/Pd alloy having a high silver content, which is relatively cheap, for forming the inner electrode.
However, the sintered body of the lead based complex perovskite composition is low in its mechanical strength. Thus, the sintered body is highly likely to be cracked in the manufacturing process of the electronic part, leading to a low product yield. Further, traces of water permeates through the cracks into the interior region of the electronic part, with the result that the metal of the electrode migrates into the dielectric layer under the action of the voltage applied to the electronic part during the operation of the electronic apparatus and the water entering the sintered body. It follows that the insulating property of the dielectric layer is lowered, leading to a low reliability of the product.
Therefore, an object of the present invention is to provide a dielectric ceramic composition which can be sintered at a relatively low temperature to provide a dielectric exhibiting a high mechanical strength and preventing water permeation even under a high humidity, and thus is capable of providing an electronic part having a high reliability.
Another object of the invention is to provide a highly reliable electronic part comprising at least one dielectric layer.